There are many known methods and catalysts useful for the homo- or co-polymerization of olefins. For example, in what are generally referred to as a traditional Ziegler-Natta catalyst systems, transition metal compounds are cocatalyzed by an aluminum alkyl to generate catalysts capable of producing both high and low molecular weight polyolefins. The polymers produced in these systems are characterized by a very broad, sometimes multimodal, molecular weight distribution and, in the case of copolymers, uneven comonomer distribution, largely due to the presence of multiple sites capable of active polymerization and variances in the chemical stability and reactivity of those sites.
Among the many varieties of Ziegler-Natta catalysts studied, a particular class of catalysts comprising transition metal compounds possessing two cyclopentadienyl ligands ("metallocenes") cocatalyzed with aluminum alkyls, are known to successfully polymerize .alpha.-olefins. However, the activity of these systems was markedly low, until the development of the now well-known metallocene-alumoxane catalyst systems. Furthermore, these alumoxane-activated systems were observed to produce polymers having narrow Polydispersity Index, or MWD (M.sub.w /M.sub.n), in contrast to traditional Ziegler-Natta systems, since the catalyst systems were largely single site catalysts.
Since that time, others have developed further these metallocene based catalyst systems. Addition of covalent bridging linkages between the cyclopentadienyl ligands of the metallocene compounds yielded monometallocyclic compounds capable of active polymerization. See, for example, EP-A-0 129 368 and U.S. Pat. No. 5,324,800. Activation of such by ionizing activators yielded saturated bimetallic catalysts see U.S. Pat. No. 5,198,401. Heterocyclic, monocyclopentadienyl transition metal compounds have been shown to be effective olefin polymerization catalysts with both of alumoxane and noncoordinating anion precursor activators, see for example U.S. Pat. No. 5,055,438 and WO-A-92/00333, which additionally can yield saturated bimetallic catalysts. Though the foregoing are largely restricted to Group 4 transition metal compounds, extension to Group 5 and 6 metal compounds was illustrated in WO-A-94/01471. Though the foregoing are largely restricted to Group 4 transition metal compounds, extension to Group 5 and 6 metal compounds was illustrated in WO-A-94/01471. Cyclic transition metal compounds comprising a Group 4 metal and two amido groups connected by a covalent bridging group containing a Group 14 or 16 element have also demonstrated catalytic activity when activated by alumoxane compounds, see U.S. Pat. No. 5,318,935.
Stable bimetallocyclic compounds of the Group 5 metals Niobium and Tantalum have been synthesized and studied as reported in the academic literature. See, "Crystal Structure of Bis-M-trimethylsilylmethylidyne)tetrakis-(trimethylsilylmethyl)diniobium(v ). A New Type of Carbon Bridging Group", Huq, Mowat, Skapski and Wilkinson, J. Chem Soc., Chem. Commun., 1477-1478(1971), and "Elimination Stabilized Alkyls. Part III. Trimethylsillylmethyl and Neopentyl Alkyls of Transition Metals", Mowat and Wilkinson, J. C. S. Dalton , 1120-1124 (1973). Similar compounds supported on silica have been reported to be active catalysts for the exhaustive hydrogenation of aromatic substrates, see "Surface-supported Group 5 Metal Organometallic Compounds for Catalytic Arene Hydrogenation", Profilet, Rothwell and Rothwell, J. Chem Soc., Chem. Commun., 42-44 (1993). Polymerization capability or activity are not reported.
In view of developments in the organometallic field that bulky ancillary ligands can be used to stabilize organometallic compounds of the early transition metals so that they will sustain a cationic charge without decomposing into catalytically inactive species, additional species capable of similar stability were sought. In particular, since each of the above systems in the patent art showed various capabilities with respect to polyolefin polymerization activities, molecular weight and microstructure capability, including levels of comonomer incorporation in polytheylene copolymers, new active but stable catalyst systems capable of single site catalysis were sought.